Gas Chromatography (Lecture 22)

Question 1

What is the mobile phase?

Answer 1

an inert gas (usually He, Ar, N₂)

the gas to be used is determined by the needs of the instrument’s detector

Question 2

What are the criteria for the stationary phase?

Answer 2

must not react with solutes
must be thermally stable
must have low volatility

non polar solvents require a non polar stationary phase
polar solvents require a polar stationary phase

Question 3

What is the most common stationary phase?

Answer 3

polydimethyl siloxane (non polar)

Question 4

What are the different types of columns?

Answer 4

packed columns

capillary columns

Question 5

What do packed columns contain?

Answer 5

They are constructed from glass, stainless steel copper or aluminium. They are filled with a particulate solid support, usually diatomaceous earth (very porous). The support acts as an active site for absorbing solute molecules.

Question 6

What are the three main types of capillary columns?

Answer 6

walled coated tubular column
porous layer open tubular column
support coated open tubular column

Question 7

What do walled coated capillary columns contain?

Answer 7

thin layer with stationary phase

Question 8

What do porous layer capillary columns contain?

Answer 8

porous solid support, thin layer

Question 9

What are support coated capillary columns?

Answer 9

They are basically the same as porous layer capillary columns but with a liquid stationary phase on a porous solid support.

Question 10

What are the factors that affect sample introduction into a GC?

Answer 10

volatility
sample conc / purity (and appropriate ratio of analytes for resolution)
sample stability (process of injection must not cause degradation)
selection of stationary phase (must have similar polarity to sample)

Question 11

How can volatility be controlled?

Answer 11

non-volatile samples (e.g. amino acids) can be reacted with 1-butanol to increase volatility

Question 12

How can sample conc be controlled?

Answer 12

extract product until a sufficient amount has been collected then dilute the product to reach an appropriate conc

Question 13

How can sample stability be controlled?

Answer 13

fragile samples that degrade easily can skip the heating phase and be directly injected into the column

Question 14

How can the stationary phase be controlled?

Answer 14

stationary liquid phase can be altered to be more or less polar to match sample

Question 15

What factors make a good detector for a GC experiment?

Answer 15

a low detection limit
a linear response over a wide range of solute concs
sensitivity for all solutes or selectivity for a specific class of solute
insensitivity to a change in temperature or flow rate

Question 16

What are the similarities between different types of GC detectors?

Answer 16

uses electrical current

Question 17

What are the differences between different types of GC detectors?

Answer 17

(Thermal Conductivity Detector - TCD)
- mobile phase exists over tungsten- rhenium wire filament
- dependent on thermal conductivity of mobile phase
(Flame Ionisation Detector - FID)
- uses a flame to create small current
(Electron Capture Detector - ECD)
- considered a selective detector
(Mass Spectrometer - MS)
- uses a magnetic field

Question 18

What is GC quantitative analysis used for?

Answer 18

used for analysis of environmental, clinical, pharmaceutical, biochemical, forensic, food science and petrochemical laboratory samples

Question 19

What is GC quantitative analysis used for?

Answer 19

identification of components in a mixture

Question 20

What is the accuracy of GC?

Answer 20

typical routine sample have an accuracy of 1-5% but as the sample conc decreases the accuracy decreases

Question 21

What is the precision of GC?

Answer 21

• based on sample preparation and instrument
• standard deviation of 1-5% usually due to detector noise
• use of internal standards compensates for variability in injection volumes

Question 22

What is the sensitivity of GC?

Answer 22

• based on the detector’s linear range (linear range = the range of conc over which a calibration curve is linear)
• wider the range, the greater the detection

Question 23

What is the selectivity of GC?

Answer 23

highly selective, you can design a detector to just separate the analyte